1. Field of the Invention
The invention relates to a method and a device for packaging crushed polycrystalline silicon material.
2. Description of the Related Art
Polycrystalline silicon (polysilicon) is usually deposited from trichlorosilane by means of the Siemens process and then, for applications in the solar industry, usually undergoes low-contamination comminution and, for applications in the semiconductor industry, comminution and subsequent partial cleaning. Depending on the planned application, the crushed polysilicon material obtained in this way may contain the maximum contaminants of metal elements stated in Table 1 after packaging.
TABLE 1Maximum content of metal contaminantsFigures given in pptwMaterialFeCrNiNaZnAlCuMoTiWKCoMnCaMgVA<50<20<10<100<20<30<10<10<100<20<100<5<20<100<100<5B<1000<100<50<1000<200<300<20<50<200<1000<200<100<20<1000<500<20A: Crushed polysilicon material for the electronics industry (after low-contamination comminution, cleaning and packaging)B: Crushed polysilicon material for the solar industry (after low-contamination comminution and packaging)
Crushed polysilicon material for the electronics industry usually has to be packaged in 5 kg bags with a weight tolerance of +/−30 g, while crushed polysilicon material for the solar industry is usually supplied in bags with an initial weight of 10 kg and a weight tolerance of +/−100 g.
Commercially available horizontal or vertical bag forming, filling and sealing machines, as are used in the pharmaceutical industry for packaging medicaments or in the food industry for packaging tea and coffee, are only suitable to a certain extent for the packaging of crushed polysilicon material, a bulk material with sharp edges that is not free-flowing and has a weight of individual Si fragments of up to 10,000 g, since this material perforates conventional plastic bags during filling and, in the worst case, completely destroys them. Moreover, it is not possible with these devices to meet the purity requirements that are required of the crushed polysilicon material in the aforementioned applications, since the composite films used lead to contaminants above the limit values stated in Table 1 on account of chemical additives, and are therefore not suitable for the packaging of crushed polysilicon material.
EP A 133 4907 (US 2005-0034430) discloses a method and a device that are intended to make it possible for high-purity crushed polysilicon material to be portioned, filled and packaged at low cost and in a fully automated manner. This device comprises a means for portioning the crushed polysilicon material, a filling device, a plastic bag, and a welding device for the plastic bag filled with crushed polysilicon material. In this filling device, the plastic bag is formed from a high-purity film of plastic by means of a filling and bag-forming tube. This procedure entails several disadvantages.
First, during the forming of the plastic bag, the plastic surface that forms the inner side of the plastic bag comes into contact with the metal surface of the filling and bag-forming tube. This leads to undesired metal contaminations of the inner bag surface. Therefore, an iron level of <50 pptw for the packaged polysilicon cannot be achieved with this device. Second, during filling of the bag with crushed polysilicon material, contact with the inner side of the filling and bag-forming tube causes contamination of the crushed polysilicon material. Third, the design-dependent high falling height of the crushed polysilicon material, or the abrasion caused by the sharp-edged crushed polysilicon material, has the effect that the plastic coating is so worn away after approximately 100 tonnes of packaged material that parts of the filling and bag-forming tube have to be exchanged. Fourth, as a result of the high falling height during filling, the crushed polysilicon material often perforates the bag wall. Fifth, an initial weight of the crushed polysilicon material within the stated tolerance is scarcely possible by means of this device.
The automatic portioning for this purpose is laborious, since the crushed polysilicon material, which generally occurs with a weight of the individual fragments of between 0.1 and 10.000 g, has to be separated into a number of product flows of differently sized fragments, which then have to be mixed together again in a specific manner ahead of the weighing balance, in order to be able to maintain the required accuracy of weight. Moreover, because of the design-dependent high falling height, this method leads to the formation of slivers and dust, and consequently to unacceptable contamination and post-comminution of the crushed polysilicon material.
On account of these disadvantages of the automatic packaging machine, labor-intensive manual packaging of the cleaned crushed polysilicon materials in a clean room of class 100 continues to be common practice for high-grade polysilicon. In the process, cleaned crushed polysilicon materials, which no longer have any metal contaminants on their surface, are taken from a process bowl, in which cleaning takes place, by someone wearing sterilized gloves, for example sterilized textile, PU or PE gloves, and are introduced into a PE double bag. Owing to glove abrasion and the general handling performed by the personnel, the content of plastic and metal particles on the crushed polysilicon material increases when it is touched with gloves. Measurements have shown that the metal surface content for the individual elements in the case of manual packaging increases on average by the values stated in Table 2:
TABLE 2Increase in contamination of crushed polysiliconmaterial in the case of manual packagingFigures given in pptwFeCrNiNaZnAlCuMoTiWKCoMnCaMgV1523151043016012001930
This shows that it is only by such laborious, time-intensive manual packaging of crushed polysilicon material that the purity requirements with respect to the metal surface values for the electronics industry are met (Table 1).